HSF1 is the master regulator of the heat shock response, in which multiple genes are induced in response to temperature increase and other stresses. At non-shock temperatures in humans and other vertebrates, HSF1 is produced constitutively, but is inactive and bound by protein HSP90. At an elevated temperature, HSF1 is released by HSP90, moves from the cytoplasm to the nucleus, and trimerizes. This active HSF1 binds to heat shock elements (HSE) in DNA and activates transcription of heat shock genes by RNA polymerase II. The HSE has a consensus sequence of three repeats of NGAAN and is present in the promoter regions of the HSP90, HSP70 and HSP27 genes. During cessation of the heat shock response, HSF1 is phosphorylated by mitogen-activated protein kinases (MAPKs) and glycogen synthase kinase 3 (GSK3) and returns to an inactive state. The biochemistry of HSF1 is described, inter alia, in Chu et al. 1996 J. Biol. Chem. 271:30847-30857; Huang et al. 1997 J. Biol. Chem. 272:26009-26016; and Morimoto et al. 1998 Nat. Biotech. 16: 833-838.
HSF1 interacts with additional factors. HSF1 binds to DNA-dependent protein kinase (DNA-PK), which is involved in DNA repair. HSF1 is a target of mitogen-activated protein kinases, and its activity is down-regulated when the RAS signaling cascade is active.
Additional heat shock factor proteins in humans include HSF2, HSF3, and HSF4. HSF1, HSF2, and HSF3 are positive regulators of heat shock gene expression, while HSF4 is a negative regulator. HSF1, HSF2 and HSF4 play a role in transcriptional control of other heat shock proteins. The various HSF proteins share about 40% sequence identity.
HSF1 has been implicated in several diseases, including cancer and viral diseases. HSF1 and other heat shock proteins (whose expression is increased by HSF1) are over-expressed in, or have otherwise been implicated in breast, endometrial, fibrosarcoma, gastric, kidney, liver, lung, lymphoma, neuroectodermal, neuroblastoma, Ewing's sarcoma, prostate, skin, squamous cell, and testicular cancers, leukemia (e.g., promyelocytic leukemia), and Hodgkin's disease.
Without wishing to be bound by any particular theory, it is believed that heat shock proteins (HSP) may block the pathways of apoptosis and permit malignant cells to arise despite the triggering of apoptotic signals during transformation. HSP expression may also afford protection to cancer cells from treatments such as chemotherapy and hyperthermia by thwarting the pro-apoptotic influence of these modalities.
Because HSF1 positively regulates HSPs, a need exists for therapeutics that modulate HSF1.